Titanium (Ti) and Ti alloys (all referred to as Ti hereafter) have many applications in aerospace, biomedical, chemical, architecture, and consumer industries. Using Ti powder as a starting material for sintering is one approach for manufacturing products from Ti. Ti powder has been in especially high demand in recent years due to the advent of additive manufacturing technologies. Ti is a promising material for additive manufacturing of metals. However, the current market size of Ti powder is very small. At least one reason for the small market is the fact that Ti powder is often very expensive.
There are a number of factors that contribute to the high cost of making Ti powder. One of these factors is that Ti powder must meet stringent requirements for low oxygen content. Oxygen in Ti metal or alloys can be detrimental to mechanical properties of the Ti metal or alloys. Higher than acceptable oxygen content in Ti can lead to low ductility, poor formability, brittleness, and potential for premature failures.
However, controlling and minimizing oxygen content in Ti is not a trivial task. Ti has strong chemical affinity to oxygen. Ti metal is easily oxidized under normal conditions. In fact, there are only a handful of elements that has stronger affinity to oxygen than Ti. Those elements include Ca, Mg, Be, Li, Ba, Al and U. In theory, these elements can be used to reduce titanium oxide, TiO2.
One challenge for making high purity and low oxygen Ti powder is to control and minimize the oxygen content in Ti powder. Methods for controlling oxygen content in Ti powder can be different depending on the specific conditions and methods used to produce and handle the powder. In some situations, when Ti powder is produced, the oxygen content in the powder may not meet the specifications, i.e., the oxygen content is higher than desired. Thus, Ti powder with higher than desired oxygen content is often subjected to a “deoxygenation” treatment. The purpose of deoxygenation is to remove oxygen from the material and reduce the oxygen content to an acceptable level. Typical requirements for oxygen content in Ti alloys in final product form can be less than 0.2%. In order to meet such a requirement, if the powder is to be used as the raw material to fabricate the product, the oxygen content in the initial powder can be less than 0.15% or 0.12% in consideration that oxygen content will most often increase during the fabrication processes.
Ti primary metal is typically produced commercially using either the industry standard batch-operated Kroll process or the Hunter processes. The Kroll process is the most dominant process today globally due to both technical as well as economic considerations. In the Kroll process, titanium tetrachloride (TiCl4) is reduced by liquid Mg to produce Ti sponge. Undesired impurities can be removed relatively easily from TiCl4 by distillation, and purified TiCl4 enables the production of highly purified Ti metal. However, the processes to produce TiCl4 involve a series of highly energy intensive and costly processes, which leads to a high price for TiCl4. Furthermore, TiCl4 is highly hazardous such that even a minor leak can cause serious damage to most metal structures and electrical equipment in the vicinity.
To avoid the drawbacks of using TiCl4, one alternative is to use commercial TiO2 as the precursor, which is safe to work with and can alternatively be produced via a sulfate process instead of the chloride process by oxidation of TiCl4. Direct use of electricity to reduce TiO2 is one option for making Ti powder from TiO2. However, the difficulties of scaling up electrolytic cells and contaminating from carbon are drawbacks to this option. Other challenges also exist in reducing TiO2. First, it is more difficult to meet the requirements for oxygen content in a final product made from TiO2 than to reduce the chlorine content of Ti made from TiCl4, due to the strong affinity of oxygen to titanium. Second, the oxide byproducts involved have much higher melting points than the chlorides produced by reducing TiCl4. Therefore, the oxide byproducts are separated from titanium by acid leaching instead of distillation. These issues continue to prevent the widespread use of TiO2 as a precursor for manufacturing Ti powder.
Titanium is known to dissolve interstitially about 33 atomic percent of oxygen. The solid solution Ti(O) includes titanium metal with dissolved oxygen atoms, which is different from titanium oxide TiO2. The process of making Ti from TiO2 can be divided into two substeps: “reduction” of reducing TiO2 to form Ti(O) through various TixOy intermediates. The oxygen content of the Ti(O) can be as high as about 14% by weight with oxygen atoms occupying octahedral interstitial sites within the Ti crystal lattice. The second substep of “deoxygenation” involves further reducing dissolved oxygen content in Ti(O) to the desired final oxygen content. Two considerations affect the cost of this process for Ti metal production: (1) most of the oxygen in TiO2 will be removed during the reduction of TiO2 to Ti(O), thus the amount of reducing agent and any other input chemicals (such as salt) will be large, and the cost of recycle or reclaiming these chemicals can be substantial; (2) the Ti—O binding energy in Ti(O) is stronger than that in rutile, and even stronger than in MgO when the oxygen content is less than 1.5% by weight. This limits the type of deoxygenation reagent that can effectively reduce the oxygen content of Ti(O). It has been reported that Ca is the only economical agent for deoxygenation.
When Ca metal is adopted, the reduction and deoxygenation steps can be merged into one step, which is named calciothermic reduction. Four different forms of Ca can be used as options for calciothermic reduction, including solid hydride CaH2, vapor-Ca, liquid-Ca, and electronically mediated reduction (EMR). The oxygen content in Ti or Ti alloys can be minimized to a very low level using Ca, for instance, 0.42% of oxygen by weight was reported in Ti metal with the assistance of CaCl2 at 900° C. by reducing TiO2. A wide range of other Ti alloys can also be prepared by calcium co-reduction of their oxide mixtures.
In addition, Ca can be applied in an independent deoxygenation process, such as the DOSS process developed by RMI Titanium. This process can include using liquid Ca as the deoxidant. This technology has been used to reduce oxygen content in β Ti alloys (for instance, Ti—Mo and Ti—V alloys). Using Ca vapor generated in vacuum as the deoxygenation agent at a relatively low temperature of 500-830° C. has also been investigated. In another method, titanium scrap with high initial oxygen content is deoxidized by mixing with Ca and CaCl2 and heating to 900-950° C. in argon, during which the CaCl2 is used to dissolve the byproduct of CaO to accelerate the oxygen removal rate. In order to avoid the impurity contamination from Ca metal, deoxygenation of Ti can also be conducted by dissolving Ca vapor in CaCl2 salt and using the chemically active Ca-saturated salt as the reducer at 1000° C.
Credited to the strong reducing ability of Ca, the various modes of calciothermic reduction and deoxygenation have been developed. However, the high operating temperature of around 900-1000° C. is a disadvantage, due to the high melting points of Ca and CaCl2.
Compared to Ca, it is traditionally believed that Mg metal can be used for preliminary reduction of TiO2 and Ca for the final deoxygenation if economics dictates such a preference, because Mg is reported to be not a strong enough reducing agent to reduce the oxygen level to the required threshold for Ti sponge, which is thought to be only effective at reducing titanium to a minimum oxygen content of 3.58% by weight at temperatures below 900° C., and thermodynamic analysis shows that there is a lower limit to the oxygen content by Mg at approximately 1.9%. Thus, the reported results of reducing TiO2 by Mg have been with an oxygen content of higher than 1% by weight.